SIROHAEM SULFITE REDUCTASE AND OTHER PROTEINS ENCODED BY GENES AT THEDSR LOCUS OF CHROMATIUM-VINOSUM ARE INVOLVED IN THE OXIDATION OF INTRACELLULAR SULFUR

The sequence of the dsr gene region of the phototrophic sulfur bacteri um Chromatium vinosum D (DSMZ 180(T)) was determined to clarify the in vivo role of 'reverse' sirohaem sulfite reductase. The dsrAB genes en coding dissimilatory sulfite reductase are part of a gene cluster, dsr ABEFHCMK, that encodes four small, soluble proteins (DsrE, DsrF, DsrH and DsrC), a transmembrane protein (DsrM) with similarity to haem-ij-b inding polypeptides and a soluble protein (DsrK) resembling [4Fe-4S]-c luster-containing heterodisulfide reductase from methanogenic archaea. Northern hybridizations showed that expression of the dsr genes is in creased by the presence of reduced sulfur compounds. The dsr genes are not only transcribed from a putative promoter upstream of dsrA but pr imary transcripts originating from (a) transcription start site(s) dow nstream of dsrB are also formed. Polar insertion mutations immediately upstream of dsrA, and in dsrB, dsrH and dsrM, led to an inability of the cells to oxidize intracellularly stored sulfur. The capability of the mutants to oxidize sulfide, thiosulfate and sulfite under photolit hoautotrophic conditions was unaltered. Photoorganoheterotrophic growt h was also unaffected. 'Reverse' sulfite reductase and DsrEFHCMK are, therefore, not essential for oxidation of sulfide or thiosulfate, but are obligatory for sulfur oxidation. These results, together with the finding that the sulfur globules of C. vinosum are located in the extr acytoplasmic space whilst the dsr gene products appear to be either cy toplasmic or membrane-bound led to the proposal of new models for the pathway of sulfur oxidation in this phototrophic sulfur bacterium.